Polyimide resin and carbon fiber molded tube clamp

ABSTRACT

A process for forming a molded polymer resin fiber tube clamp, as well as the tube clamp formed by that process. Continuous top and bottom plies provide increased resistance to delamination/cracking as they sandwich a filler material. The tube clamp is free from exposed fiber ends so than no wear is produced on a tube being clamped.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of allowed U.S. patent application Ser.No. 09/613,162 filed Jul. 10, 2000, now U.S. Pat. No. 6,841,021 which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to gas turbine engines, and morespecifically, to an improved clamp for securing tubes within the engineagainst movement, including vibratory movement.

BACKGROUND OF THE INVENTION

A gas turbine engine such as that used for powering an aircraft inflight includes, for example, numerous tubes for channeling variousfluids through the engine during operation. Clamps are used for mountingthe tubes to the engine casing, for example, at standoff brackets toaccurately position the tubes and prevent their movement duringoperation of the engine.

Since the gas turbine engine includes various rotating components, thetubes are subject to vibratory excitation which must be controlled forpreventing vibratory fatigue damage to the tubes. Conventional tubeclamps used in an aircraft gas turbine engine typically have two clamphalves pivoted together at respective first ends so that the clamphalves may be opened for inserting one or more tubes between the halves,the halves then pivoted together to capture the tubes. Each clamp halfincludes a generally semi-circular recess which collectively surround arespective tube, and a fastener hole that extends through the tube clamphalves so that a suitable fastener, such as for example, a bolt, may beinserted through the holes, with a complimentary nut joined to the boltfor clamping together the tube clamp halves around one or more tubescontained therein. The fastener typically also extends through anengine-mounted bracket for joining the tube clamp and the tubes to theengine casing.

An example of a second prior art tube clamp design includes a base plateor lower clamp half, having a flat lower surface disposed on a supportplate, a flat upper surface having an arcuate, semi-circular, firstrecess for receiving the tube, and a first hole spaced laterally fromthe first recess and extending through the base plate from the lower toupper surfaces. A capture plate or upper clamp half is positioned abovethe base plate and includes a lower surface facing the base plate uppersurface, an upper surface, and a second hole extending through thecapture plate from the lower to upper surfaces.

Clamps of the character indicated are used in aircraft construction forsupport of tubing in various environments, involving, for example,relatively great longitudinal displacement as in the course of wingflexure or lesser longitudinal displacements as in the case of vibratoryoscillation, and elevated temperature as in the vicinity of an engine.The aircraft structure experiences high vibration levels, temperaturevariations, aerodynamic buffeting, and structural flexure. Often, a wearsleeve extends around the tubing, a clamp extends around the wearsleeve, and the clamp is coupled to the engine wall. The clamp isloosely coupled around the wear sleeve so that when the tubing movesrelative to a wall, the wear sleeve moves axially within the clamp.

Tube clamps used within gas turbine engines are typically made ofsuitable metals such as for example, aluminum, stainless steel, orInconel which are selected for use in the engine depending upon thetemperature of the individual location, from relatively cool near thefan of the engine to relatively hot near the combustor and turbines.Metal tube clamps are known to abrade or chafe the tubes containedwithin them due to vibratory excitation of the tubes during engineoperation, so that a conventional wear sleeve made of, for example,epoxy is positioned between the tube and the tube clamp to preventundesirable wear of the tube during operation.

Because metal is known to be a poor vibration damper, metal tube clampsprovide little vibratory damping of the tubes contained within them, andthese tubes are subject to vibratory excitation during operation of theengine. Wear sleeves associated with these tube clamps are an additionalpart that must be suitably secured to the tube to prevent theirliberation during operation of the engine, which is undesirable.

To combat these problems, polymer resin and fiber composite tube clamps,for example, polyimide resin and carbon fiber composite are currentlybeing utilized in tube clamp design. Sheets or plies of compositeprepreg, referred to as prepreg, are laid up into large flat plates ofdifferent thickness and cured. Clamps are then subsequently machinedfrom these plates to meet various required configurations.

Composite tube clamps are characterized by the absence of discreet wearsleeves surrounding the tubes as would be required in metal tube clamps.A composite tube clamp will, therefore, have significant weight savingsover a metal design and will also have inherent vibratory dampingcapability significantly greater than that which is obtainable frommetal tube clamps. This damping reduces the vibratory energy in thetubes and increases the useful life of these tubes.

Composite tube clamps may be formed of conventional polymeric resinsthat are commercially available. For example, clamps used in the coolerregions of the engine near, for example, the fan may be made from lowtemperature resins like epoxy or bismaleimide (BMI). In hotter regionsof the engine, polyimide resin composites such as PMR-15 polyimidematrix resin developed by the NASA Lewis Research Center and AMB-21, maybe used due to their higher temperature capability. Furthermore,structural fibers may be used in a matrix for providing selectivestrength of the tube clamp. Conventional fibers, such as, for example,fiberglass or carbon fibers or polymeric fibers have been used in asuitable resin matrix such as those disclosed above. The fibers havebeen oriented at random or they have been aligned for obtainingadditional strength.

These composite clamps are manufactured by laminating a plurality ofprepreg sheets or plies into large flat plates of different thickness,then machining the plate into strips, and subsequently machining fromthese strips clamps to meet various predetermined configurations. Themachining process exposes fiber ends; these machined ends of the fiberwhich contact the tube abrade the tube causing it to wear locally underthe clamp.

Another inherent problem with a machined composite clamp is that due toits laminar design, when the clamp is installed, it frequentlydelaminates or cracks as a result of the load necessary to assemble it.This cracking and delamination occurs, for example, between plies in alocation of maximum flatwise tension due to bending, for example, aswould be caused by tightening of the fastening mechanism holding theupper and lower clamp halves against the tube. In actual use,installation failure rates of up to thirty three percent have beenreported.

What is needed is a composite tube clamp that can take advantage of theweight saving properties of composite, utilizing the strength of a fiberreinforcement, thus significantly increasing the delamination/crackingresistance, while at the same time, not exposing abrasive fiber ends tocreate wear on a tube contained within the clamp.

SUMMARY OF THE INVENTION

The present invention provides a process for manufacturing a polymerresin fiber composite tube clamp, and the tube clamp formed by thatprocess, which takes advantage of the weight savings of composites, andthrough use of a net molded fiber reinforcement, significantly increasesthe delamination/cracking resistance, and at the same time, does notexpose abrasive fiber ends which could create wear on a tube containedwithin the clamp.

In one form, the present invention is a process for forming a tube clampcomprising the steps of layering curable fiber reinforced material tothe contour of layup tooling having a preselected shape, curing thematerial and removing the cured material from the layup tooling after ithas cured, retaining the preselected contours of the layup tooling toyield the desired contours for the tube clamp without exposing thefibers.

In another form, the present invention comprises a process for forming atube clamp comprising the steps of layering curable, fiber-reinforcedmaterial to preselected contours of first layup tooling having adesired, predetermined shape, layering curable, fiber-reinforcedmaterial to the contours of second layup tooling having a desired shape,curing the material, removing the cured material from the first andsecond layup tooling after the fiber-reinforced material has cured whileretaining the contours of the first and second layup tooling, and matingthe cured layering material from the first and second layup tooling toform a net shape or near net shape article.

In yet another form, the present invention comprises forming a pluralityof articles such as tube clamps as a single article in a single lay-upand curing operation as described above, edge trimming the curedmaterial and then slicing or cutting the edge trimmed cured materialinto desired widths corresponding to that of the article.

In yet other embodiments, the present invention includes articles in theform of tube clamps made by the processes described above.

An advantage of the present invention is that the tube clamp will bemolded to net or near net shape using fiber reinforced composite, sothat machining of the clamping surface is eliminated. This moldingapproach will take advantage of the strength of the fiber reinforcement,thereby significantly increasing the delamination/cracking resistance,reducing the installation failure rate, while reducing production costs.

Another advantage of the present invention is the ability to dispensewith the use of a wear sleeve. Because the fibers are continuous,maintaining the contour of the clamp surface, there are no exposed fiberends with which to wear or abrade a clamped tube. Therefore, no wearsleeve is required; consequently, production costs are significantlyreduced.

Yet another advantage of the present invention is that this processexposes no fiber ends that can act as an abrasive on the enclosed tube.Thus, the source of tube wear, abrasive fiber ends exposed by subsequentmanufacturing operations such as machining, also is eliminated.

Still another advantage of the present invention is because there are noexposed fiber ends to abrade the tube, the need for a wear sleevesurrounding the tube is eliminated, thus reducing production costs andsimplifying assembly.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a profile view of the prior art machined clamp;

FIG. 2 is a view of prepreg material on the layup tooling; and

FIG. 3 is a view of the present invention after removal of the layeringmaterial from the layup tooling.

DETAILED DESCRIPTION OF THE INVENTION

For purposes of the present invention, the word “cured” means a processby which a flexible material is made rigid.

The present invention can be better appreciated by comparison to theprior art machined composite resin fiber tube clamp. FIG. 1 displays aprior art machined composite resin fiber tube clamp comprising an upperhalf 2 a lower half 4 and bolt 6 mating the upper half to the lowerhalf. Each half of the prior art clamp is comprised of laminatedcomposite plate assembled from prepreg sheet and cured. After curing,its contours are machined to retain a tube contained therein and slicedinto strips of appropriate width. The machining process exposes fiberends 8. These fiber ends 8 are not only abrasive to the tube (not shown)being encased by the prior art tube clamp, the machining process, incutting the fibers 10, destroys the continuity of the fibers 10,particularly in the region of the tube, thus reducing the strength acontinuous fiber imparts to the clamp's structural integrity.

When load is applied to the enclosed tube (not shown) upon tightening ofthe bolt 6, the prior art clamp will often delaminate and/or crackbetween plies 12 in the location of maximum flat wise tension in thearea of the cut, non-continuous fibers 14 in the vicinity of the tube.

Referring now to FIGS. 2 and 3, the process for forming the improvedtube clamp 20 of the present invention comprises the steps of layeringcurable fiber reinforced prepreg plies or sheet 22 to the contour oflayup tooling 24 having a predetermined shape, removing the material 22from the layup tooling 24 after the material 22 has been cured to a netshape or near net shape article while retaining the contour of the layuptooling 24. As used herein, the term prepreg ply is used interchangeablywith the term prepreg sheet.

In the preferred embodiment, tube clamp 20 is comprised of a compositeof fibers in a polymer resin matrix. Composite articles made up fromfilms, sheets or plies are easy to design, produce, standardize, andcontrol. In the present invention, polyimides are the polymer of choice,due to their ability to handle temperatures in excess of 650° F., anadvantage in the hot sections of an aircraft engine. The hot sections ofan aircraft gas turbine include the combustor and those portions aft ofthe combustor including the turbine sections. Other polymer resins thatmay be utilized include, for example, acetylene-terminated polymers suchas, for example, acetylene-terminated quinoxalines, polyamide-imides,phthalocyanines, polyesters, epoxies or any other polymer resinproviding high strength and stiffness. To gain additional structuralstrength, in the present invention carbon fibers are imbedded in thepolymer, although other fibers such as, for example, fiberglass fibers,aramid fibers, metal fibers, ceramic fibers including for example,silicon carbide fibers, thermoplastic fibers or any other fibers such asfor example, glass or glass/ceramic fibers including alumina, sapphireand silica may also be used. The fibers may be non-coated but frequentlyare coated for various reasons, such as to improve bonding to the matrixmaterial or to protect the fiber. The only limitation on fiber selectionis based on the ability of the fiber to withstand the loads and thetemperatures of the selected application. In certain applications, yarncomprised of fibers may be used. Additionally, a ceramic fiber embeddedin ceramic matrix (CMC), such as SiC/SiC composite may be utilized.

The fibers may be imbedded in the matrix material as uni-directionallyoriented fibers in a tape or ply, fibers running bidirectionally inwoven cloth, or mat having random fiber orientation. The uni-directionalply provides excellent strength in the direction parallel to the tape,while woven cloth fibers provide excellent strength in the plane of thewoven cloth. Polymer infiltrated into a fiber mat having randomlyoriented fibers, hereinafter also referred to as random fiber mat, isnot as strong as the unidirectionally oriented plies or the woven clothplies in their preferential directions of maximum strength, but itsstrength is substantially uniform in all directions.

Layup tooling 24 is fabricated by standard fabrication processes toproduce a pattern that is a negative image of the final shape desired inthe improved tube clamp 20. All corners in the layup tooling 24 arerounded, having a radius of about 0.05 inches to about 0.3 inches,preferably about 0.1 inches. Although square corners may be utilized,radiused corners are preferred so that stress risers are eliminated.Layup tooling 24 shown in FIG. 2 depicts only the pattern having thedesired contour of a tube clamp. The layup tooling for such a tube clampincludes a plurality of straight sides that mate with the depictedpattern to provide a mold for containment. The uncured compositematerial must be contained, as the matrix material has a tendency toflow upon heating. Opposite the depicted pattern is a movable platenthat can be used to apply pressure to compress the material as it isheated. The mold and the movable platen, usually made of a metal, form amatch metal press. In an alternate embodiment, the platen may bereplaced with a flexible caul if the laid up material is to be cured inan autoclave. It will be understood that if a complex configuration ofthe net shape or near net shape article is required, a mold havingstraight sides can be replaced by a mold in which each of the sidesincludes predetermined contours of the final product.

In another embodiment, prepreg ply is placed in intimate contact alongthe contour pattern of layup tooling 24. Additional plies are thenlayered in the tooling until a predetermined thickness is achieved(collectively referred to as “bottom ply layer 34”). When plies havingunidirectional fiber are layed up, the plies can be layed up so that thefibers are parallel, or, if desired, alternating plies can be placed sothat the fibers are angled at predetermined angular orientations fromadjacent plies depending on the stresses that the clamp will experiencewhen additional strength is required. For example, plies may be laid upin a pattern starting at 0°, then 45°, then 90°, then 45°, then 0°(0°/45/90°/45°/0°). The desired thickness of the bottom ply layer 34 isabout 0.01 inches to about 0.3 inches, preferably about 0.06 inches.

A top ply layer 38 is formed in a similar manner. The desired thicknessof the top ply layer is about 0.01 inches to about 0.3 inches,preferably about 0.06 inches. Both the top ply layer 38 and the bottomply layer 34 can be formed of unidirectional prepreg sheets, wovenprepreg sheets or random fiber mat. Prepreg sheets comprisinguni-directional fiber and woven fabric are thinner than random fiber matthat tends to be thicker. As a result, fewer mats containing choppedfiber are required to achieve the predetermined thickness than would berequired using uni-directional prepreg sheets or woven cloth prepregsheets. The bottom ply layer 34 and top ply layer 38 provide additionalstrength for beam bending.

One of the variations of this embodiment utilizes additional fillerplies placed between the top and bottom ply layers. Referring to FIG. 2,these plies may not all be of the same shape, as some filler plies 31may be cut to size to appropriately fill the regions in the mold betweenthe bottom and top ply layer. In another variation, to reduce productioncosts, as ply cutting and ply layup is labor intensive, filler materialmay be sandwiched in the mold between the top ply layer 38 and thebottom ply layer. This filler material 26 may be, for example, polyimideresin, chopped fiber molding compound, plies cut to shape, or othertypes of fillers such as, for example, polyimide ceramic foams, ormixtures of these filler, if desired. In this manner, void regionsbetween the top ply layer and the bottom ply layer are filled, forexample, with a mix of polymer resin and chopped fiber. As previouslynoted, the number or shape of filler ply layers is determined by theclamp size or shape required for a specific application, but the numberused can be reduced by substituting filler for some or all of the plies.The determination as to whether to substitute plies with a differentfiller is primarily determined by the mechanical properties required ofthe tube clamp, as generally, a tube clamp laid up completely from plieshas superior mechanical properties in directions parallel to the plies.

Typically, each ply of polyimide carbon fiber composite mat is about 15to 50 mils in thickness, preferably about 20-30 mils, and mostpreferably about 25 mils. The plies are pre-impregnated with fibers in arandom pattern to save on production costs; however, in situations whereincreased strength is required, uni-directional pre-impregnated fiberplies may be used. These plies have a thickness of from 5-15 milsdepending on the size of the fiber or fiber bundle. Optionally,pre-impregnated fiber plies can be replaced by using techniques such asresin transfer molding. Curing at an elevated temperature using apreselected pressure at a preselected temperature is preferred. This canbe accomplished in an autoclave by laying a caul over top ply layer 38.Alternatively, a movable metal platen can be assembled over top plylayer 38 and pressure can be applied to the material within the mold asit is heated.

Once the material within the mold is cured, such as, for example, byautoclaving or by utilizing a match metal press technique, the moldedmaterial will maintain the shape of the layup tooling 24 as shown inFIGS. 2 and 3. If the sidewalls of the mold also include contours orfeatures, then the molded material will also include these features.Optionally, the edges 28 of the removed cured layers may be trimmed toprovide a smoother surface as required. If required, the cured materialmay then be sliced into strips 40 by for example, a cut off wheel,router, or grinder, to provide a clamp of required width for a specificapplication. Clamping holes 30, as required, may then be machinedutilizing standard machining techniques.

As shown in FIG. 2, because the plies are layered to the contour asdetermined by the layup tooling, no machining of the clamp assembly isnecessary (other than the optional edge trimming to provide smootheroutside edges). Because there are no machined tube contours, there areno exposed fiber ends to abrade and cause wear on the clamped tube.Additionally, because no fibers in areas of high stress have been cut bya machining process, fibers run throughout the contour of the tubeclamp, so that discontinuous fibers are eliminated in these criticalareas, thereby providing additional strength for bending. As shown inFIG. 2, when unidirectional or woven fibers are used to form the bottomply layer 34 and the top ply layer 38, the fibers are continuous andunbroken, so that strength is not compromised in these critical areas.This structure is better able to transmit loads by elimination of weakerregions where high stresses can exceed the local yield stress of thematerial. Typically, these high stresses occur between the plies and arethe cause of delamination and the source of cracks. The continuous fiberin this region provides a significant increase in both the delaminationstrength and cracking strength in these regions.

The process of the present invention may be utilized to form twoseparate halves of a tube clamp, which are then mated together. Thepresent invention comprises both the processes for manufacturing thetube clamp as described above, as well as the tube clamp that resultsfrom the above-described process.

The improved molded clamp design of the present invention utilizingmolded polyimide resin carbon fiber was compared to a productionmachined polyimide resin carbon fiber tube clamp. A displacing force wasapplied to a sample on a hydraulic press, measuring load and deflectionuntil failure. Failure was indicated by large deflections withoutcommensurate increases in load.

The machined surface sample failed at 1008 lbs. and at 1013 lbs. Theimproved molded tube clamp of the present invention failed at 3200 lbs.and 3400 lbs., showing significant increase in the load bearing abilityof the improved molded tube clamp design.

Because of the ability to transmit loads uniformly across the structure,the articles made in accordance with the present invention haveincreased load bearing ability so that there is less clamp failureduring installation from cracking and delaminating, thereby furtherreducing production costs.

Although the present invention has been described in connection withspecific examples and embodiments, those skilled in the art willrecognize that the present invention is capable of other variations andmodifications within its scope. These examples and embodiments areintended as typical of, rather than in any way limiting on, the scope ofthe present invention as presented in the appended claims.

1. A tube clamp for use in a hot section of a gas turbine engine,comprising: a first half of cured composite material, the first halfhaving a first portion, a second portion, and a filler material, thefirst portion including fibers embedded in a matrix, the fiberscomprising axial surface portions and comprising fiber ends defined byradial surface portions, the first portion being of a predeterminedthickness to form a bottom ply layer and having a predetermined contourconforming to at least a portion of a tube and an interface, the bottomply layer having a contoured tube-contacting surface comprising axialsurface portions of the fibers and essentially no radial surfaceportions, the second portion being of a predetermined thickness to forma top ply layer, the filler material being sandwiched between the topply layer and the bottom ply layer; a second half of cured compositematerial, the second half having a first portion, a second portion, anda filler material, the first portion including fibers embedded in amatrix, the fibers comprising axial surface portions and comprisingfiber ends defined by radial surface portions, the first portion beingof a predetermined thickness to form a bottom ply layer and having apredetermined contour conforming to at least a portion of a tube and aninterface corresponding to the interface of the first half, the bottomply layer having a contoured tube-contacting surface comprising axialsurface portions of the fibers and essentially no radial surfaceportions, the second portion having fibers embedded in a matrix, thesecond portion being of a predetermined thickness to form a top plylayer, the filler material being sandwiched between the top ply layerand the bottom ply layer; and means for joining the first half to thesecond half.
 2. The tube clamp of claim 1 wherein the matrix is apolyimide resin.
 3. The tube clamp of claim 1 wherein the fibers arecarbon fibers.
 4. The tube clamp of claim 1 wherein the filler materialis formed from random mat fiber.
 5. The tube clamp of claim 1 whereinthe filler material is formed from prepreg sheet having oriented fibers.6. The tube clamp of claim 1 wherein the filler material is formed fromchopped fibers and polyimide resin.